The
Baal Shem Tov, or Besht — the founder of Chasidism —
met the soul of the Messiah during an ascent to heaven.
The Besht asked him, "When will the Master come?"
The Messiah answered, "When your wellsprings break forth to the outside!"
(from a letter
written by the Besht to his brother-in-law about one of his soul ascents)

Birkat Ha-ilanot

The month of Nisan is filled with remarkable moments (including the Pesach seder), but one of the moments that touches me the most deeply is the blessing of the fruit trees. This blessing is traditionally said when one is in view of at least two flowering fruit trees, anytime through the end of the month of Iyyar (or in Tishrei down under—in a climate where you can say it in Nissan it's considered best to do so). It's also a beautiful thing to teach at your Tu Bish'vat seder. The brakhah goes like this:

"Blessed be You, Yah our God ruler of space and time, for God left nothing lacking in God's world, and created in it good creatures and good trees, giving pleasure through them to the children of Adam."

The language of the blessing recalls part of the borei n'fashot blessing for food, which thanks God for creating "many souls and all their lacks" chesronam, yet here we say that there is nothing missing or lacking. One could say that the things we lack are themselves the essence of creation, calling us to weave relationships with all forms of life. The gift of fruit, however, embodies an even greater sense of pure abundance and blessing than almost anything else we encounter.

We have a unique intimacy with fruit trees. In scripture that goes back to Gan Eden and the tree of knowing. The connection is even more powerful in the midrashic interpretation of the statement in Deuteronomy 20, Ha'adam eitz ha-sadeh, "A person is a tree of the field" (that is, a fruit tree). (The statement in context is really a question.) For Kabbalah, a fruit tree is as true an image of God as a person (see below as well as the blessing from P'ri Eitz Hadar). The Sefirot, "the Tree of Life", are thought of as a fruit tree. The reason why is that a fruit tree embodies the principle of sharing, and is a more perfect model for how God interacts with the world than human beings can be.

Why do we need to see two trees rather than just one to say the blessing? I haven't heard an explanation, but one reason is that the trees need each other to reproduce, at least on the species level (most fruits—except dates and a few others that are gendered by tree—can also fertilize themselves). The halakhah specifically forbids saying the blessing over trees that are grafted from one species onto another – there is an idea of appreciating the awesome reality of this world in itself, separate from human "chokhmas" and power.

Here are some quotes about fruit trees from Tanakh, midrash and Kabbalah:

This one, your body, was like a palm tree, and your breasts clusters [of dates].
I said, I will climb up that palm tree, I will grab its branches.
May your breasts be like clusters [of grapes] on the vine, the scent of your breathing like apples.
And your mouth like good wine, going straight to my lover, lubricating sleepers' lips.
I am my lover's, and his desire is upon me.
Song of Songs 7:8-11

R' Yishma'el said: The compassion of the Place Maqom מקום [God] is on the fruit of the tree....For if scripture cautions you [not to harm] the tree that makes fruit [Deut. 20:19], all the more so the fruits themselves. Sifrey Deuteronomy Pisqa 203

R' Abba taught: There is no greater revelation of redemption than that which the verse states: "And you, mountains of Israel, you shall give forth your branches and you shall bear your fruit for my people Israel, for they shall soon come." [Ez. 36:8] Talmud Bavli Sanhedrin 98a

When R' Abba saw a tree whose fruit turned into a bird and flew away, he wept and said: If men only knew to what these things alluded, they would rend their garments! Zohar 2:15b